A biaxially oriented polyester film such as a polyethylene terephthalate (PET) film is widely utilized because of its excellent thermal and dimensional stability, good mechanical properties, etc. Biaxially oriented film is oriented machine directionally and transverse directionally, hereinafter referred to as MD and TD. Such film is very useful as the base film of magnetic tapes and condensers. The stronger the mechanical properties, the thinner the film that can be used for the magnetic tapes, condensers, and the like, with the result that the equipment which uses such tape can be made smaller and lighter, or can offer substantially higher performance if the size remains the same.
A longitudinally tensilized PET film has been used as a thin base film. The film is produced by an MD post-stretching of the ordinary biaxially oriented film. However, the MD tensilized film has the disadvantage of poor roll formation when wound as a roll film since MD wrinkles occur frequently.
Because of rapid development in the field of information technology there is a great need for a thinner base film. To fulfill this need, the thinner film must have enhanced mechanical properties in both MD and TD. A film which has stronger mechanical property only in MD is not sufficient. Ordinary biaxially oriented polyester film has a MD and TD F-5 value (a tensile strength at 5% elongation) of 10-11 kg/mm.sup.2. MD tensilized film has good MD attributes, but the F-5 value in TD is only 9-11 kg/mm.sup.2, so that the thinner the film becomes, the more unsuitable it is.
We have already applied for Japanese Patent No. 74-39808, wherein we proposed a method for producing a polyester film that is stronger in both MD and TD as a result of a re-stretching in TD of the longitudinally post stretched biaxially oriented polyester film. However, since the film is free in TD there remained the problem of shrinkage and neckdown in TD when the biaxially oriented film is restretched in MD on an ordinary roll system.